1. Field of the Invention
The present invention relates to switching power supply units for supplying DC stabilized voltages. More specifically, the present invention relates to a switching power supply unit which operates in a current-continuation mode by allowing two switching elements to alternately turn on/off so as to perform self-excited oscillation.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 11-187664 discloses a switching power supply unit, in which first and second switching elements Q1 and Q2 are connected on the primary side of a transformer T, switching control circuits for alternately turning on/off the first and second switching elements, with a period when both the first and second switching elements are turned off therebetween, are provided, energy is stored in a primary winding and an inductor L during an ON-period of the first switching element Q1, the energy is emitted from a secondary winding during an OFF-period of the first switching element Q1, and the first and second switching elements Q1 and Q2 perform self-excited oscillation. The switching power supply unit having this configuration is called a flyback self-excited oscillation switching power supply unit including two switching elements.
Other examples of flyback switching power supply units including two switching elements are disclosed, for example, in Japanese Unexamined Patent Application Publication No. 4-87560, Japanese Unexamined Utility Model Registration Application Publication No. 6-36392, and PCT Japanese Translation Patent Publication No. 10-500834. The flyback switching power supply units disclosed in these publications have the same configuration as that of the switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-187664 such that two switching elements are provided on the primary side of the transformer T. However, the two switching elements of these switching power supply units do not perform self-excited oscillation. In addition, these switching power supply units operate in a current-continuation mode. In the current-continuation mode, a current is applied to the secondary side of a transformer T and is then applied continuously to the primary side without a pause such that the waveform of the current applied to the primary switching element is trapezoidal. On the other hand, a current-discontinuation mode includes a pause such that a current is applied to neither the primary side nor the secondary side, and the waveform of the current applied to the primary switching element is triangular.
However, the above-described known switching power supply units have the following defects.
Next, the defects of the flyback self-excited oscillation switching power supply unit disclosed in Japanese Unexamined Patent Application Publication No. 11-187664 will be described.
In this switching power supply unit, the waveform of the current applied to the primary winding is always triangular, such as in ringing choke converters. Accordingly, a peak current on the primary side increases under heavy load and the effective current increases. When an effective current increases, copper loss of the transformer and conduction loss of the switching elements increase, and thus, efficiency decreases. This prevents miniaturization of a switching power supply unit.
Furthermore, since the waveform of the current applied to the primary winding is triangular, an ON-time of the switching element increases under heavy load. In addition, an OFF-time also increases in accordance with the extension of the ON-time. Accordingly, the switching period increases and the switching frequency decreases. As a result, a large transformer, a smoothing capacitor on the secondary side, and other components are required, which prevents miniaturization of a switching power supply unit.
Next, defects of the flyback two-element switching power supply unit which operates in a current-continuation mode disclosed in Japanese Unexamined Patent Application Publication No. 4-87560 will be described.
The waveform of the current applied to the primary winding is trapezoidal, and thus, copper loss of the transformer and conduction loss of the switching elements is reduced. However, since this type of switching power supply unit does not perform self-excited oscillation, a high-pressure-resistance drive IC or a pulse transformer is required for driving an oscillation circuit, a totem-pole circuit for driving, and an upper switching element at a different ground level, as a drive control circuit for alternately turning on/off the two switching elements. As a result, miniaturization and cost reduction for a switching power supply unit cannot be sufficiently achieved.
In order to overcome the above-described problems, preferred embodiments of the present invention provide a highly efficient switching power supply unit having greatly reduced size and weight and which is produced at a greatly reduced cost.
Preferred embodiments of the present invention provide a new technique for combining the operation characteristic of the known flyback two-element switching power supply unit operating in a current-continuation mode with a self-excited oscillation method. The switching power supply unit of various preferred embodiments of the present invention includes the following combination and arrangement of elements.
According to a first preferred embodiment of the present invention, a switching power supply unit operating in a current-continuation mode includes a transformer including a primary winding, a secondary winding, a first drive winding, and a second drive winding, an inductor, a first switching circuit connected in series to a series circuit including the primary winding and the inductor, the first switching circuit including a parallel circuit having a first switching element, a first diode, and a first capacitor, an input power supply connected in series to the series circuit, a second switching circuit connected in parallel to the series circuit, the second switching circuit including a parallel circuit having a second switching element, a second diode, and a second capacitor, a capacitor connected in parallel to the series circuit, a rectifying and smoothing circuit which includes a rectifying element and which is connected to the secondary winding, and first and second switching control circuits for alternately turning on/off the first and second switching elements, with a period when both the first and second switching elements are turned off therebetween, the first switching control circuit being connected between the first drive winding and a control terminal of the first switching element and the second switching control circuit being connected between the second drive winding and a control terminal of the second switching element.
Energy is stored in the primary winding and the inductor during an ON-period of the first switching element and the energy is emitted from the secondary winding during an OFF-period of the first switching element such that the first and second switching elements perform self-excited oscillation. The inductor and the capacitor define a resonance circuit which resonates during the OFF-period of the first switching element. The first switching control circuit includes a first ON-time control circuit in which a time constant is set such that the first switching element is turned off at a desired time after the first switching element is turned on. The second switching control circuit includes a second ON-time control circuit in which a time constant is set such that the second switching element is turned off in order to interrupt a resonance current applied to a series circuit including the second switching element and the inductor after the second switching element is turned on and before the energy is completely emitted from the secondary winding.
With this configuration, the first ON-time control circuit and the second ON-time control circuit defining the first and second switching control circuits operate in a different manner. In a flyback self-excited oscillation switching power supply unit as disclosed in Japanese Unexamined Patent Application Publication No. 11-187664, a second ON-time control circuit for controlling a second switching element Q2 on the primary side of a transformer T turns off the second switching element Q2 after energy is completely emitted from a secondary winding. On the other hand, in preferred embodiments of the present invention, the second ON-time control circuit forces the resonance current applied to the series circuit having the second switching element and the inductor to be interrupted after the second switching element is turned on and before the energy is completely emitted from the secondary winding. That is, in the second ON-time control circuit, a desired time constant is set such that this operation is performed.
According to the second ON-time control circuit, the second switching element is turned off so as to interrupt the current applied to the inductor before the energy is completely emitted from the secondary winding. Due to this change in current, the voltage at the primary winding is reversed, and thus, a voltage is generated at the first drive winding so as to turn on the first switching element. Accordingly, a self-excited oscillation operation is performed. Also, a current-continuation mode, in which a current is applied to the secondary side of the transformer and the current is continuously applied to the primary side without a pause, is achieved such that the waveform of the current applied to the first switching element on the primary side is trapezoidal. That is, the operation is performed in a current-continuation mode in which the waveform of the current applied to the first switching element under heavy load is trapezoidal. Thus, the peak current applied to the transformer and the first switching element and the effective current is greatly reduced, copper loss of the transformer and conduction loss of the first switching element is greatly reduced, and thus, a compact, lightweight, and highly efficient switching power supply unit is achieved.
According to a second preferred embodiment of the present invention, a switching power supply unit operating in a current-continuation mode includes a transformer including a primary winding, a secondary winding, a first drive winding, and a second drive winding, an inductor, a first switching circuit including a parallel circuit having a first switching element, a first diode, and a first capacitor, an input power supply, a second switching circuit connected in parallel to the first switching circuit, the second switching circuit including a parallel circuit having a second switching element, a second diode, and a second capacitor, a capacitor connected in parallel to the first switching circuit, a rectifying and smoothing circuit which includes a rectifying element and which is connected to the secondary winding, and first and second switching control circuits for alternately turning on/off the first and second switching elements, with a period when both the first and second switching elements are turned off therebetween, the first switching control circuit being connected between the first drive winding and a control terminal of the first switching element and the second switching control circuit being connected between the second drive winding and a control terminal of the second switching element. The primary winding, the inductor, the first switching circuit, and the input power supply are connected in series.
Energy is stored in the primary winding and the inductor during an ON-period of the first switching element and the energy is emitted from the secondary winding during an OFF-period of the first switching element such that the first and second switching elements perform self-excited oscillation. The inductor and the capacitor define a resonance circuit which resonates during the OFF-period of the first switching element. The first switching control circuit includes a first ON-time control circuit in which a time constant is set such that the first switching element is turned off at a desired time after the first switching element is turned on. The second switching control circuit includes a second ON-time control circuit in which a time constant is set such that the second switching element is turned off in order to interrupt a resonance current applied to a series circuit including the second switching element and the inductor after the second switching element is turned on and before the energy is completely emitted from the secondary winding.
In this configuration, the location for connecting the capacitor is different from that of the first preferred embodiment of the present invention. However, the operation is the same as in the first preferred embodiment, and thus, a compact, lightweight, and highly efficient switching power supply unit is achieved. Further, although the voltage applied to the capacitor is greater than in the first preferred embodiment, the capacitance is reduced when a desired charge is stored. Accordingly, the size of the capacitor is greatly reduced.
According to a third preferred embodiment of the present invention, a switching power supply unit operating in a current-continuation mode includes a transformer including a primary winding, a secondary winding, a first drive winding, and a second drive winding, an inductor, a capacitor, a first switching circuit including a parallel circuit having a first switching element, a first diode, and a first capacitor, an input power supply, a second switching circuit connected in parallel to a series circuit including the primary winding, the inductor, and the capacitor, the second switching circuit including a parallel circuit having a second switching element, a second diode, and a second capacitor, a rectifying and smoothing circuit which includes a rectifying element and which is connected to the secondary winding, and first and second switching control circuits for alternately turning on/off the first and second switching elements, with a period when both the first and second switching elements are turned off therebetween, the first switching control circuit being connected between the first drive winding and a control terminal of the first switching element and the second switching control circuit being connected between the second drive winding and a control terminal of the second switching element. The primary winding, the inductor, the capacitor, the first switching circuit, and the input power supply are connected in series.
Energy is stored in the primary winding and the inductor during an ON-period of the first switching element and the energy is emitted from the secondary winding during an OFF-period of the first switching element so that the first and second switching elements perform self-excited oscillation. The inductor and the capacitor define a resonance circuit which resonates during the OFF-period of the first switching element. The first switching control circuit includes a first ON-time control circuit in which a time constant is set such that the first switching element is turned off at a desired time after the first switching element is turned on. The second switching control circuit includes a second ON-time control circuit in which a time constant is set such that the second switching element is turned off in order to interrupt a resonance current applied to a series circuit including the second switching element and the inductor after the second switching element is turned on and before the energy is completely emitted from the secondary winding.
In this configuration, the location for connecting the capacitor is different from that of the first preferred embodiment of the present invention. However, the operation is the same as in the first preferred embodiment, and thus, a compact, lightweight, and highly efficient switching power supply unit is achieved. Also, in this configuration, the primary side of the transformer has a so-called half-bridge configuration. Therefore, the voltage applied to the first and second switching elements is equal to the input voltage, and thus, the applied voltage decreases as compared to that in the first preferred embodiment of the present invention. In general, the ON-resistance of a low-pressure-resistance switching element is small, and thus, the conduction loss due to the ON-resistance is reduced and high-efficiency is achieved. Also, the voltage applied to the transformer is about one half that of the first preferred embodiment, and thus, the number of windings is reduced so as to achieve a compact and highly efficient transformer.
Also, during the OFF-period of the first switching element, the electrostatic energy stored in the capacitor is emitted as well as the exciting energy stored in the transformer during the ON-time of the first switching element. Accordingly, the peak current applied to the transformer and the switching element is greatly reduced, thus greatly reducing the effective current and conduction loss.
Preferably, the switching power supply unit further includes an overcurrent protection unit including an overcurrent protection circuit which includes a current detecting unit connected in series to the first switching element and which limits an ON-time of the first switching element when the current applied to the first switching element detected by the current detecting unit reaches a threshold.
In this configuration, since the overcurrent protection circuit for detecting the peak current applied to the first switching element and limiting the current is provided, saturation of the transformer and destruction of the switching elements caused by an increase in the peak current during overcurrent and at startup is prevented.
The overcurrent protection circuit includes a third switching unit for turning off the first switching element, the third switching unit being connected to the control terminal of the first switching element, and the overcurrent protection circuit turns on the third switching unit when a peak current applied to the current detecting unit reaches the threshold so as to turn off the first switching element.
With this arrangement, the third switching unit limits the peak current of the first switching element, and thus, the configuration of the overcurrent protection circuit is greatly simplified.
The first switching control circuit includes a first delay circuit which includes a series circuit including a resistor or a resistor and a capacitor and which is connected between the first drive winding and the control terminal of the first switching element. Also, the second switching control circuit includes a second delay circuit which includes a series circuit including a resistor or a resistor and a capacitor and which is connected between the second drive winding and the control terminal of the second switching element. The first delay circuit delays a voltage which is generated at the first drive winding and which turns on the first switching element so as to delay the turn on of the first switching element. Also, the second delay circuit delays a voltage which is generated at the second drive winding and which turns on the second switching element so as to delay the turn on of the second switching element.
By providing the delay circuits, a turn-on timing of the switching elements is delayed, and each of the switching elements are turned on when the voltage applied to the switching element is decreased to zero or close to zero. Accordingly, a zero-voltage switching operation is performed and switching loss is greatly reduced.
Also, in the known art, in which the waveform of the current applied to the first switching element is triangular, turning off the rectifying element on the secondary side is a trigger for the resonation between the inductor and the first and second capacitors and for turning on the first switching element. On the other hand, in the preferred embodiments of the present invention, turning off the second switching element while the energy stored in the inductor is emitted is a trigger for generating a voltage at the primary winding and turning on the first switching element. With this operation, the resonance period when the voltage is reversed is shorter as compared to the circuit in the known art, and thus, the delay time for turning on the first switching element is reduced.
Further, the resistor defining the delay circuit attenuates a voltage surge generated at the drive winding and delays the rise time of the control voltage so as to delay the turn-on time. The voltage at the capacitor is divided with the input capacitance of the switching element such that the voltage applied to the control terminal can be adjusted.
A delay time is set in the first delay circuit such that the first switching element is turned on when a voltage applied across the first switching element is decreased to zero or close to zero, and a delay time is set in the second delay circuit such that the second switching element is turned on when a voltage applied across the second switching element is decreased to zero or close to zero.
With this configuration, the first and second switching elements perform zero-voltage switching by the first or second delay circuit, and thus, switching loss is further reduced.
The first ON-time control circuit includes a first switching unit for turning off the first switching element and turns on the first switching unit so as to turn off the first switching element at a desired time after a voltage for turning on the first switching element is generated at the first drive winding.
Accordingly, the output voltage is stabilized by the first ON-time control circuit including the time-constant circuit for the first switching element.
The second ON-time control circuit includes a second switching unit for turning off the second switching element and turns on the second switching unit in order to turn off the second switching element after a voltage for turning on the second switching element is generated at the second drive winding and before the energy is completely emitted from the secondary winding, whereby a current applied to the series circuit including the second switching element and the inductor is interrupted.
With this arrangement, the second ON-time control circuit including the time-constant circuit for the second switching element turns off the second switching element before the energy is completely emitted from the secondary winding of the transformer. This is a trigger for reversing the voltage generated at the transformer and generating a voltage at the first drive winding, and the first switching element is turned on with this voltage such that the first switching element performs self-excited oscillation.
Accordingly, as described above, the waveform of the current applied to the first switching element is trapezoidal and the current-continuation mode is performed.
Values of the inductor and the capacitor are set such that a resonance current applied to the inductor and the capacitor reaches a peak when a current applied to the inductor is interrupted by turning off the second switching element by the second switching control circuit.
In this configuration, by turning off the second switching element when the resonance current is close to its peak, the capacitance of the capacitor is reduced and miniaturization of the capacitor is achieved.
Also, with the resonance current applied to the inductor and the capacitor, the waveform of the current applied to the rectifying element on the secondary side is an upward sine-wave. Accordingly, the peak current is reduced and the effective current is reduced.
Further, by turning off the second switching element after the resonance current reaches the peak current, the turn-off current of the rectifying element on the secondary side is reduced. Accordingly, recovery loss of the diode is reduced.
The first ON-time control circuit includes a circuit for varying a time until the first switching element is turned off according to a signal corresponding to an output voltage.
In this configuration, the time until the first switching element is turned off is shortened under light load according to a signal corresponding to the output power, and the time until the first switching element is turned off is increased under heavy load. Accordingly, the output voltage is stabilized.
Preferably, the third switching unit includes a transistor connected to the control terminal of the first switching element, the voltage generated at the current detecting unit is applied to a control terminal of the transistor through a resistor, the transistor is turned on when the current applied to the first switching element reaches a desired value and the voltage at the control terminal of the transistor reaches a threshold, and the first switching element is turned off so as to limit the peak current applied to the first switching element.
By defining the third switching unit by the transistor, the divided voltage of the voltage generated at the current detecting unit is compared with a threshold of the transistor (base-emitter voltage: about 0.6 V). Accordingly, the peak current of the first switching element is limited, the number of components is reduced which results in a simplified configuration. Also, a low cost, compact, and lightweight switching power supply unit is achieved.
The voltage generated at the first drive winding during the ON-period of the first switching element is input to the control terminal of the transistor through a resistor and a diode.
When the input voltage varies, if the peak current is constant, the overcurrnet point increases as the input voltage increases. At this time, by inputting the voltage which is generated at the first drive winding and which is proportional to the input voltage to the control terminal of the third switching unit through the resistor and the diode, the overcurrent point is lowered only when the input voltage is high, and thus, the variation in the overcurrent point due to the variation in the input voltage is suppressed. That is, the third switching unit is turned on earlier when the input voltage is high. This contributes to achieving a compact and lightweight switching power supply unit.
The overcurrent protection unit includes a first ON-time limiting unit for defining the maximum ON-time of the first switching element and a second ON-time limiting unit for turning off the first switching element when the current applied to the first switching element reaches a desired value, the first and second ON-time limiting units being independent from each other.
The peak current is limited by the second ON-time limiting unit when overcurrent occurs, and the maximum ON-time of the first switching element is reduced by the first ON-time limiting unit when the output voltage decreases. Accordingly, an increase in the output current on the secondary side is suppressed, and a short-circuit current is reduced.
Preferably, at least one of the first switching element and the second switching element includes a field-effect transistor.
With this arrangement, the parasitic capacitance of the field-effect transistor is used as the first capacitor or the second capacitor. Also, a parasitic diode of the field-effect transistor is used as the first diode or the second diode. Accordingly, the number of components is greatly reduced, and a low-cost, compact, and lightweight switching power supply unit is achieved.
Preferably, the inductor includes a leakage inductor included in the transformer.
By using a leakage inductor included in the transformer as the indictor, the number of components is further reduced, and a low-cost, compact, and lightweight switching power supply unit is achieved.
Further, a capacitive impedance is preferably connected across the rectifying element.
By connecting a capacitive impedance across the rectifying element on the secondary side, the recovery loss of the rectifying element is greatly reduced, and thus, a high-efficiency and low EMI noise are achieved.
Other feature, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.